Fixed minor bugs in API classes.

  * DataFlow: Fixed typo in public method name.
  * FlowStats: Fixed minor bug in toString().

Other changes:

  * Refined unit tests for org.opendaylight.vtn.manager.flow package.

Change-Id: I6060ab23bd83614bb9110339b6b21195b4a34226
Signed-off-by: Shigeru Yasuda <s-yasuda@da.jp.nec.com>